Most of the pultruded fiber reinforced plastic products are made from unsaturated polyester resin and epoxy resin. Phenolic resin which is known to have excellent mechanical properties and electrical properties, especially at elevated temperatures, has been used to produce fiber reinforced products by prepreging, compounding or reaction injection molding processes, but phenolic resin used as a binder in a pultrusion process is not considered to be feasible due to its slow curing rate, and due to the side product (e.g. water) which may be generated causing void and defects.
U.K. patent No. 1,363,227 discloses a modified phenolic resole resin prepared from a phenol-aldehyde condensation product, a glycol and an acid catalyst, in which the glycol is presented in the final product in an amount of about 12 to 35% by weight based on weight of the phenol-aldehyde condensation product. The modified phenolic resole resin has an improved curing rate at a lower curing temperature.
U.S. patent 4,419,440 discloses fiber reinforced phenolic resin products prepared by a pultrusion process, in which a liquid phenolic resin, essentially the same as the modified phenolic resole resin taught by above said U.K. patent, having a viscosity in the range of about 500 to about 4,000 centipoises at 23.degree. C. is used to impregnate the filaments. The spirit of the invention disclosed by this U. S. patent as stated in lines 10 to 14 of column 4 in its specification is "The low viscosity aids in processing the liquid phenolic resin and filament strands through the extrusion step in the production of filament-reinforced products in the invention." Therefore the liquid phenolic resin used is maintained at a relative low degree of cross linking and is modified in order to have the low viscosity, i.e. 500 to 4,000 cps at 23.degree. C., preferably 800 to 3,000 cps. Consequently, the heating temperature of the pultrusion die is kept at a relative high temperature and the pulling rate of resin-filament composite is limited to a relative low value in order to cure the in-situ the phenolic resin to a sufficient extent. Furthermore, if the pulling rate is higher than the limit value, water contained in the liquid phenolic resin as a side product of the phenol-aldehyde condensation reaction will quickly expand right after the resin-filament composite exits from the heated pultrusion die and causes the pultruded products to become hollow or full of voids. Example 3 of U.S. Pat. No. 4,419,400 is found to have a pulling rate of 12 inch/minute.
The present invention is a process for pultruding fiber reinforced phenolic resin products comprising the steps of drawing a plurality of continuous filaments through an impregnating bath of liquid phenolic resin to wet-out the filaments with said resin and a squeeze orifice for removal of excess resin and air, and continuously pulling the resin-filament composite through a pultrusion die to heat and cure said resin, characterized in that the liquid phenolic resin normally has a viscosity ranging from 3,000 to 5,000 cps at 25.degree. C. and is heated to have a viscosity ranging from 800 to 2,000 cps during the impregnating step.
The viscosity of liquid phenolic resin used in the present pultrusion process has a value significantly higher than the one suggested by U.S. patent 4,419,400 which means the phenolic resin used in the present invention has been crossed linked to a higher degree, which permits the resin impregnated filament to be sufficiently cured under less severe conditions compared to the process of U.S. patent 4,419,400, i.e. under a lower curing temperature and/or a higher pulling rate. Use of a phenolic resin hasing a viscosity ranging from 3,000 to 5,000 cps, preferably 3,500 to 5,000 cps, is not possible for impregnating the filaments according to the teaching of U.S. patent 4,419,400. The phenolic resin used in the present process is heated for obtaining a lower viscosity during the impregnating step to wet-out the reinforcement filaments. Accordingly, a process for pultruding fiber reinforced phenolic resin products disclosed by the present invention will have a pulling rate higher than any known process.
Advantageously, the pultrusion die used in the present process is designed to have multiple different heating temperatures along the pultrusion directions, e.g. three heating zones, in which the first heating temperature. i.e. near the entrance of the pultrusion die, is kept at a temperature high than 100.degree. C. in order to repel water contained in the phenolic resin and the third heating temperture is kept at a temperature lower than the second heating temperture to prevent from the water vaporizing and expanding inside the pultruded products at the exit of the pultrusion die.
Additionally, a postcuring treatment may be optionally employed to phenolic resin products for improving their physical and mechanical properties, which comprises heating the pultruded products at a temperature of about 100.degree. C. for more than 12 hours or a temperature of about 200.degree. C. for about 1-2 hours to achieve the best results.